Fly-by-night
Part of speech: noun
Definition: A creatures which flies at night; a nocturnal flier or traveler. One who departs or flees at night in order to avoid creditors, law enforcement etc. (often used attributively). Businesses that appear and disappear rapidly, or that give an impression of transience. Traveling businessmen and tradesmen.
Untrustworthy
Part of speech: adjective
Definition: Not deserving of trust; unreliable.